Zeolite is a generic name for aluminosilicates having relatively large pores in its crystal structure. Zeolite has the following structure. That is, zeolite has a basic framework including silicon dioxide. A portion of the silicon is substituted with aluminum. Thus, the basic framework of zeolite is negatively charged. The charge is balanced by cations such as alkali metal contained in micropores. Zeolite is typically represented by general formula xM2O.yAl2O3.zSiO2.nH2O (M is cationic metal such as alkali metal). Zeolite has a variety of characteristics depending on the type of a cation contained in the crystal.
The crystal structure of zeolite is classified based on structure codes by International Zeolite Association. For example, it is known that a plurality of crystal structures, such as faujasite, mordenite, and chabazite, exist. Chabazite (also called a chabazite structure) as one of the crystal structures of zeolite has a three-dimensional pore structure including an eight-membered oxygen ring. This crystal structure effectively acts in various uses such as catalysts and adsorbents. A zeolite having a chabazite structure is called a chabazite zeolite.
U.S. Pat. No. 6,709,644 discloses that a chabazite zeolite can be used for the separation of gas, the selective reduction of nitroxides contained in the exhaust gas from automobiles, the conversion of oxygen-containing hydrocarbon such as lower alcohol into liquid fuel, and a catalyst and a separation membrane for the manufacture of dimethylamine.
A chabazite zeolite is sometimes used as an active component of an automobile exhaust gas purification catalyst. In this case, a substrate such as a honeycomb body is usually coated with a chabazite zeolite together with an inorganic binder component (such as silica sol and alumina sol) by dip coating, wash coating, or the like. Exhaust gas flows through this substrate at a high space velocity (SV=about 10,000 to 100,000 hr−1). Therefore, if an active component-containing coat layer of an automobile exhaust gas purification catalyst does not have sufficient strength, the coat layer peels. When the coat layer peels, the active component decreases. As a result, catalytic activity decreases. Thus, the chabazite zeolite used as an active component of an automobile exhaust gas purification catalyst is required not to easily peel from a substrate.
Japanese Patent No. 5499042 and Japanese Patent No. 5683111 disclose a chabazite zeolite used as an active component of an automobile exhaust gas purification catalyst. However, these chabazite zeolites easily peel from a substrate, coated with the chabazite zeolites, of an automobile exhaust gas purification catalyst. Therefore, there is a problem in that the chabazite zeolites do not have sufficient durability when the substrate has been coated.